This invention relates to coil components and composite coils therefor, mainly intended for the control of common-mode noise in power supply input circuits of desktop electronic apparatus such as notebook type computers, word processors, and game machines, especially personal computers.
The applicant proposed in JP-A-10-22140 (U.S. patent application Ser. No. 08/884,940) to make it possible to mount pot-core components in planar position by modifying these components into a structure wherein a bobbin that carries windings is fitted in a pot core half, coil terminals of the windings are led out of through holes or through grooves, and a pot core half is mounted on them (Japanese Utility Model Application Kokai No. 5-66922) or by modifying a structure wherein a grooves formed in the rim of a pot core, through which terminals are led out and then a plate cover core is joined to the pot core (Japanese Utility Model Application Kokai No. 59-4602 1).
Namely, the above-mentioned application provided, as illustrated in FIG. 19, a coil component comprising a pot core 5 having a bottom 3 in which through holes 4 are formed, an inner leg 1 at the center and outer leg 2, a coil retained in the pot core 5, and a cover core 11 joined to the open end of the pot core 5, characterized in that said coil has terminals 8a and 8b retained in the through holes to such manner that the lower ends thereof do not protrude beyond the bottom, and said bottom has membrane external electrodes formed on the outer surface thereof and connected with the terminals 8a and 8b by filling a solder in the holes.
The prior art technique enabled planar mounting of mount coil parts with terminals such as power sources which require a large current passage, whereby the mounting is facilitated, the cost for manufacturing is reduced and the electronic devices are made compact. However, there still remain following difficulties.
In the coil part or component disclosed in JP-A-10-22 140, inner coil 6a and outer coil 6b are wound in this sequence about an inner leg 1 of a magnetic pot core as shown in FIG. 17, there is a difference in length between the inner and outer coils 6a and 6b, so that the inductance components of the conductors are larger for the outer coil than the inner coil. Secondly, the distances from the coils to the inner leg which forms a main path of the magnetic flux of the pot core are different and thus the magnetic resistance of the outer coil is larger than the inner coil. Since the effect on self-inductance of the magnetic resistance of the conductor is larger than that of the inductance component of the conductor, the self-inductance of the outer coil is smaller than that of the inner coil because the outer coil has larger magnetic resistance though the inductance of the conductor is larger while the inner coil has smaller magnetic resistance though the inductance of the conductor is smaller. As a total result, the inner coil has a larger impedance than the outer coil. The difference in these properties results in the difference in terminal noise voltage of the electronic devices in which the coil part is used. In other words, the part is directional in the properties. The directional part requires control of manufacturing processes and uses due to this directional nature and this must be taken into consideration when the circuits on circuit boards are designed.
In order to solve the problems of the prior art, the present invention controls the inductance components of the conductors by adjusting the lengths of the inner and outer coils in such manner that the inductance component of the inner coil is made small as much as possible and that of the outer coil is made larger as much as possible. In addition, a gap is preferably provided between the inner leg of the magnetic core and the inner coil to increase the magnetic resistance of the inner coil due to the leakage of the magnetic flux into the gap, whereby the self-inductance of the inner coil is decreased. In other words, the present invention utilizes as shorter a length of the inner coil as possible to reduce the inductance of the conductor of the inner coil, preferably assisted with a gap between the inner coil and the inner leg of the pot core. At the same time, the present invention utilizes as longer a length of the outer coil as possible to increase the inductance of the conductor of the outer coil by forming a gap between the inner coil and the outer coil.
The present invention provides a self-standing composite coil consisting of an inner coil and an outer coil with a gap between the inner coil and the outer coil. The length of the outer coil is made longer while that of the inner coil is made shorter, so that the conductor length of the inner coil is made shorter to make the inductance of the inner coil smaller, while the conductor length of the outer coil is made longer by a length determined by the gap between the inner coil and the outer coil to make the inductance of the outer coil larger, whereby the unbalance between the two coils is compensated for with respect to their self-inductances. Preferably, by providing a gap between the inner coil and the inner leg of the pot core, the inductance of the inner coil is further reduced to make it easier to equalize or make closer the inductances of the inner and outer coils.
The present invention further provides a coil component comprising a pot core having a bottom and through holes formed in the bottom, a composite coil retained in the pot core, and a cover core joined to the rim of the pot core, said composite coil having self-standing or shape-retaining terminals inserted in the through holes to such an extent that their lower ends do not protrude beyond the bottom, the bottom having membrane external electrodes formed on its outer surface and connected with the terminals with solder filled in the through holes. The composite coil is characterized in that the composite coil consists of an inner coil wound around the inner leg of the pot core and an outer coil wound around the inner coil and a gap is formed between the inner and outer coils so as to make larger the length of the outer coil than the conventional outer coil which was wound directly on and around the inner coil. This construction equalizes or makes closer the inductances of the inner and outer coils.
It is preferred to select the lengths of the inner and outer coils as well as the gap between the inner and outer coils so that the difference in the inductances of the inner and outer coils falls within about 10%.
More preferably, the lengths of the inner and outer coils as well as the gap between the coils are so selected that the inductances of the inner and outer coils are the same or almost the same.
The gap between the inner and outer coils is at least as large as the diameter of the coils which is the same for coil conductors or wires of both coils.
Preferably, a gap is also formed between the inner leg of the pot core and the inner coil, whereby the inductances of the inner and the outer coils are made further closer.
The present invention further relates to a shape-retaining composite coil consisting of an inner coil and an outer coil wound around the inner coil characterized in that a gap is formed between the inner and outer coils so as to make larger the length of the outer coil than the conventional outer coil. The lengths of the inner and outer coils as well as the gap between the coils are preferably so selected that the difference in the inductance between the inner and outer coils is within about 10%. More preferably, the lengths of the inner and outer coils as well as the gap between the coils are so selected that the inductances of the inner and outer coils are the same or almost the same.
The coil component and the composite coil according to the present invention are particularly effective for common mode noise suppression. That is, the composite coil and the coil component composed from the composite coil according to the present invention exhibit a high impedance against the common mode noise (synchronous signal) and a high suppression effect on the emission noise (at 30 MHz to 1 GHz) is attained. Also, suppression of noise for each line at the noise terminal voltage (at 150 KHz-30 MHz) is attained depending on the line impedance.
If there is a large difference in impedance or inductance between the lines, one line emits more noise than the others. The conventional method to overcome this problem was to add a circuit for noise suppression such as LC filters or the like on the circuit board. The present invention suppresses the emission of noise and eliminates the addition of such filters by making smaller or eliminating the difference in the impedance between the inner and outer coils.